Interpretive Summary: Babesia equi is a tick transmitted parasite which infects the red blood cells of horses. When the number of infected red blood cells is very high during the acute phase of infection large numbers of cells can be destroyed; this may result in disease, and sometimes death. If the horse survives the initial infection it remains chronically infected at a low level for the rest of its life. The parasite also undergoes a developmental cycle in ticks, and infection of ticks is required for transmission to occur. International movement of chronically infected horses is restricted because it is presumed that ticks can become infected and transmit the parasite when they feed on these chronic carriers. However, tick-borne transmission from low level chronically infected horses to uninfected horses has not been demonstrated, and the proportion of ticks that become infected and are capable of transmission has not been quantified. This information is important for understanding how this parasite is maintained and spread in horse populations. We infected horses with B. equi, then waited until they had become chronically infected. Then we fed nymphs of the most important worldwide vector of B. equi, the cattle fever tick Boophilus microplus, on the chronically infected horses. After the ticks had molted to the adult stage we tested to see if they could transmit the parasite to uninfected horses. We also tested the ticks to see how many had infected salivary glands, suggesting that they would be able to transmit the parasite when they fed. In several groups of ticks fed on chronically infected horses, from 7 to 50% had infected salivary glands. Ticks fed on horses during acute infection fell within this same range. Ticks that became infected when feeding on chronically infected horses, as well as those fed during acute infections, were able to transmit the parasite when re-fed in uninfected animals. These results demonstrate without a doubt that horses with low level chronic infections can serve as sources of infection for transmission to uninfected animals.

Technical Abstract:
The protozoan parasite Babesia equi replicates within erythrocytes. During acute infection B. equi can reach high levels of parasitemia, resulting in hemolytic crisis. Horses that recover from acute disease remain chronically infected. Subsequent transmission is dependent upon the ability of vector ticks to acquire B. equi and, following development and replication, establishment of B. equi in the salivary glands. Although the restriction of movement of chronically infected horses with B. equi is based on the presumption that ticks can acquire and transmit the parasite at low levels of long-term infection, parasitemia levels during chronic infection have never been quantified, nor has transmission been demonstrated. To address these epidemiologically significant questions, we established long-term B. equi infections (> 1 year), measured parasitemia levels over time, and tested whether nymphal Boophilus microplus ticks could acquire and, after molting to the adult stage, transmit B. equi to naïve horses. Babesia equi levels during chronic infection ranged from 10^3.3 to 10^6.0 per ml of blood with fluctuation over time within individual horses. Boophilus microplus ticks fed on chronically infected horses with mean parasite levels of 10^5.5+0.48 per ml of blood acquired B. equi with detection of B. equi in the salivary glands of 7-50% of fed ticks, a range encompassing the percentage of positive ticks that had been identically fed on an acutely infected horse with high parasitemia levels. Ticks that acquired B. equi from chronically infected horses, as well as those fed during acute infection, successfully transmitted the parasite to naïve horses. The results unequivocally demonstrated that chronically infected horses with low level parasitemia are competent mammalian reservoirs for tick transmission of B. equi.